Ron Dennis, the chairman of McLaren, is set to resign his post today, against the backdrop of the continuing investigation into the team's actions at the Australian grand prix. His decision not to travel to China for this weekend's race was thought to be simply for the launch of the company's road car; instead he is believed to be also addressing staff at their Woking headquarters to inform them of his decision.

Dennis, who is 61, stepped down as McLaren's team principal on 1 March, in favour of Martin Whitmarsh, with his long-standing protégé Lewis Hamilton the world champion. Since then little has gone right for a team who have won 10 of their 12 formula one drivers' championships since Dennis's arrival in 1980.

Their 2009 car has proved slower than those of many of their rivals, especially Jenson Button's Brawn team, who yesterday won their appeal over the controversial diffusers that have contributed to their driver's victories in the season's first two races.

Against the odds Hamilton crossed the line in the opening first race in fourth and was subsequently promoted to join Button in the first three in Melbourne when Jarno Trulli was docked 25 seconds for overtaking while the safety car was on the track. However, tapes of conversations with the pit lane revealed that Hamilton had been instructed to allow Trulli past; an investigation concluded that McLaren had provided "evidence deliberately misleading to the stewards" and as a result Hamilton was disqualified.

The driver has since claimed that he was told not to tell the stewards of the instructions by Dave Ryan, the team's sporting director, who was subsequently sacked. The FIA, the sport's governing body, is continuing its investigations into the affair, with McLaren due to face a meeting of the World Motor Sport Council in Paris on 29 April. Further sanctions are likely.

Dennis, who was in the pit lane in Melbourne, owns 15% of the company and will not be ending his association with McLaren entirely. Instead he will say that he is concentrating on the road car. However, the announcement will conclude a career in formula one that began in 1966, when he first worked for the Cooper formula one team as an 18-year-old.

Meanwhile Button's chances of extending his lead in the drivers' championship this weekend were enhanced when an FIA appeal court confirmed that his car is legal. Brawn, along with Toyota and Williams, were given the all-clear after an eight-hour hearing on Tuesday examined claims by Ferrari, BMW Sauber and Red Bull that the aerodynamic diffusers on the rear of the three cars in question broke the rules.

The decision by the FIA means that other teams will now have to incorporate the "double-deck" diffuser as quickly as possible, a task that is particularly difficult for Ferrari and Red Bull given the construction of their cars.

All five will have made some small adjustments by the time practice begins on Friday for the third round of the championship but none, at this early stage, will have been able to claw back the performance advantage enjoyed by the appellants, particularly Brawn. That said, their car relies on more ingenious thought for its superior handling and grip than just the diffuser, which creates downforce by the way air is extracted and expelled from beneath the rear of the car.

The verdict was no surprise given that the FIA's scrutineers, more conversant with the technical rules than any appeal court, had passed the cars in Australia and Malaysia. Yet Ross Brawn, the man behind the car bearing his name, endured criticism from Nigel Tozzi, Ferrari's lawyer. Tozzi described Brawn, who helped Ferrari win six straight constructors' titles, as a "person of supreme arrogance".

Brawn was not drawn in by the attack any more than the five judges were swayed by it. "We are pleased with the decision reached by the International Court of Appeal today," Brawn said. "We respect the right of our competitors to query any design or concept used on our cars through the channels available to them. The FIA technical department, the stewards at the Australian and Malaysian grands prix and now five judges at the International Court of Appeal have confirmed our belief that our cars have always strictly complied with the 2009 technical regulations." "The decision of the International Court of Appeal brings this matter to a conclusion and we look forward to continuing on the track the challenge of what has been a very exciting start to the world championship."

Brawn's views were echoed by Toyota and Williams and underlined a verdict for common sense even if the FIA's curious methods of governance allowed the matter to drag on and create yet another negative image for the sport.

While Button and Brawn are likely to revel in their technical advantage at the Chinese grand prix this weekend in Shanghai – a circuit with similar demands to Sepang –the judgment is another blow for Ferrari and McLaren, who have struggled to make their cars work with or without the contentious diffuser.

Renault are less badly affected but still at a disadvantage. "It would be possible to have something done to our car by the first race in Europe, the Spanish grand prix on 10 May," said Pat Symonds, their director of engineering. "That could find us maybe three tenths of a second. But to find the other two or three tenths necessary to put us on a par with the three cars carrying what I believed to be an illegal diffuser will require a substantial reworking of the rear of the car. Ironically, the three would have had a much easier job converting the other way."